Answer:
a) 
b) 
Explanation:
Given:
mass of the lighter block, 
velocity of the lighter block, 
mass of the heavier block, 
velocity of the heavier block, 
a)
Using conservation of linear momentum:

where:
 final velocity of the lighter block
 final velocity of the lighter block
 final velocity of the heavier block
 final velocity of the heavier block

 ........................(1)
 ........................(1)
Since kinetic energy is conserved in elastic collision:



divide the above equation by eq. (1)
 .............................(2)
 .............................(2)
now we substitute the value of v from eq. (2) in eq. (1)



 (negative sign denotes that the direction is towards left)
 (negative sign denotes that the direction is towards left)
b)
now we substitute the value of v' from eq. (2) in eq. (1)



